Transition delay by means of a passive mechanism*

Reducing the skin friction is important in nature and in many technological applications when larger speeds or lower energy consumptions are sought for. This reduction may be achieved by reducing stresses in turbulent boundary layers for instance tailoring biomimetic rough skins. Here we take a second approach consisting in keeping the boundary layer laminar as long as possible$\footnote{Joslin, R. D. 1998 {\it{Annu. Rev. Fluid Mech.}} {\bf{30}}, 1--29}$. We report the results of wind-tunnel experiments, motivated by previous theoretical analyses$\footnote{Cossu \& Brandt 2002 {\it{Phys. Fluids}} {\bf{14}}, L57--L60.}$$^{,}$$\footnote{Cossu \& Brandt 2004 {\it{Eur. J. Mech./B Fluids}} {\bf{23}}, 815--833.}$ in which a well controlled spanwise periodic modulation of the boundary layer thickness is induced by using suitably designed$\footnote{Fransson, Brandt, Talamelli \& Cossu 2004 {\it{Phys. Fluids}} {\bf{16}}, 3627--3638.}$$^{,}$$\footnote{Fransson, Brandt, Talamelli \& Cossu 2005 {\it{Phys. Fluids}} {\bf{17}}, 054110.}$ roughness elements placed on the skin. We show, both with smoke visualization and measurements, that using this passive control technique it is possible to sensibly delay transition to turbulence.